FIG. 1 shows a conventional design of a back-projector 1, the optical axis of which is centered on the screen, said back-projector comprising an objective 13, two plane deflection mirrors 11 and 12 and a screen 10, which is likewise plane. In this design, the illumination beam 14 emitted by the projector is folded by the deflection mirrors 11 and 12. The system is symmetrical with respect to a plane normal to the screen 10, the optical axis 15 of the beam being in this plane. The projector 1 may have a depth p of up to 45 centimeters for a screen 10 having dimensions of 1106 by 622 millimeters. The aperture angle along the diagonal of the screen must be about 38°. A distortion and an MTF (Modulation Transfer Function) that are acceptable may be obtained with an objective of moderate cost consisting of about ten or so lenses.
Another design is to fold the beam twice, as shown in FIG. 2. The back-projector 2 illustrated in this figure comprises an objective 23 and two plane deflection mirrors 21 and 22 placed facing each other and parallel to a screen 20. The axis of the projection objective 23 is not perpendicular to the centre of the screen 20. It is thus possible to reduce the depth p1 (for example about 20 cm) of the back-projector. However, the height h1 of the lower part of the back-projector 2 (that is to say the part located below the screen) remains large.
Patent Application EP 1203977 filed by Mitsubishi® describes several embodiments of video back-projectors comprising an aspheric deflection mirror, which makes it possible to reduce the optical aberrations and the overall size of the projector. FIG. 3 illustrates a back-projector 3 comprising an objective 33, a first plane deflection mirror 31, an aspheric deflection mirror 32, a second plane deflection mirror 36 and a screen 30. Since the objective 33 makes an angle of about 60° with the normal to the screen, the emitted beam 34 is firstly deflected in a direction normal to the screen 30 by the mirror 34 onto the aspheric mirror 32. The beam is then folded once by the mirror 36 before reaching the screen 30. Thus, the overall size is reduced. However, the back-projector 2 has the drawback that it still remains bulky.